Method for decoding image and apparatus using same

ABSTRACT

A method of decoding a image according to an embodiment of the present invention, which supports a plurality of layers, may comprise the steps of: receiving information on a reference layer used to decode a current picture for inter-layer prediction; inducing the number of valid reference layer pictures used to decode the current picture on the basis of the information on the reference layer; and performing inter-layer prediction on the basis of the number of valid reference layer pictures.

TECHNICAL FIELD

The present invention relates to video encoding and decoding, and moreparticularly, to methods and apparatuses for encoding and decoding avideo supporting a plurality of layers in a bitstream.

BACKGROUND ART

In recent years, as high definition (HD) broadcast services arespreading domestically and globally, a large number of users are gettingused to high-resolution and high-quality videos and accordinglyinstitutions put spurs to the development of next-generation videodevices. Also, with growing interest in ultrahigh-definition (UHD)services having a resolution four times higher than HDTV, compressiontechniques for higher-quality videos are needed.

For video compression, there may be used an inter prediction techniqueof predicting pixel values included in a current picture from temporallyprevious and/or subsequent pictures of the current picture, an intraprediction technique of predicting pixel values included in a currentpicture using pixel information in the current picture, or an entropyencoding technique of assigning a short code to a symbol with a highappearance frequency and assigning a long code to a symbol with a lowappearance frequency.

Video compression technology may include a technique of providing aconstant network bandwidth in restricted operating environments ofhardware without considering variable network environments. However, tocompress video data used for network environments involving frequentchanges of bandwidths, new compression techniques are required, whereina scalable video encoding/decoding method may be employed.

DISCLOSURE Technical Problem

An aspect of the present invention is to provide a method of signalinglayer information contained in a video encoded bitstream of a multilayerstructure including a temporal layer, an interlayer prediction methodand a method of obtaining a target output layer.

Another aspect of the present invention is to provide a method ofaccessing layer information specified in a video parameter set (VPS) ina bitstream for session negotiations without an entropy decoder and anapparatus using the same.

Still another aspect of the present invention is to provide a method ofidentifying a number of active interlayer reference pictures needed fordecoding a current picture for utilization in interlayer prediction, amethod of obtaining a target output layer and an apparatus using thesame.

Technical Solution

An aspect of the present invention provides a method of decoding a videosupporting a plurality of layers, the method including receivinginformation on a reference layer used for decoding a current picture forinterlayer prediction; deriving a number of active reference layerpictures used for decoding the current picture based on the informationon the reference layer; and performing interlayer prediction based onthe number of active reference layer pictures.

All slices of the current picture may have the same number of activereference layer pictures.

When a layer identifier of a current layer including the current pictureis 0, the number of active reference layer pictures may be derived to be0.

When a number of direct reference layers of the current layer includingthe current picture is 0, the number of active reference layer picturesmay be derived to be 0.

When a number of reference layer pictures, derived based on a number ofdirect reference layers of the current layer, maximum temporal sub-layerinformation of a reference layer, maximum allowed value of temporalsub-layer allowing inter-layer prediction in the reference layer and atemporal identifier of the current picture, in the same access unit asthat of the current picture is 0, the number of active reference layerpictures may be derived to be 0.

When a layer identifier of the current layer including the currentpicture is 0 or a number of reference layer pictures available forinterlayer prediction in the same access unit as that of the currentpicture is not 0, and all direct reference layer pictures belonging toall direct reference layers of the current layer including the currentpicture, being present in the same access unit as that of the currentpicture and being included in an interlayer reference picture set of thecurrent picture are used as reference layer pictures for the currentpicture, the number of active reference layer pictures may be derivedbased on a variable indicating a number of direct reference layers ofthe current layer, maximum temporal sub-layer information on each layer,maximum allowed value of temporal sub-layer allowing inter-layerprediction in each layer and the temporal identifier of the currentpicture.

A number of pictures in a reference layer having maximum temporalsub-layer information greater than or equal to the temporal identifierof the current picture and maximum temporal sub-layer informationallowing interlayer prediction greater than the temporal identifier ofthe current picture, among direct reference layer pictures for thecurrent picture, may be used as the number of active reference layerpictures for decoding the current picture.

When interlayer prediction is not used for decoding the current picture,the number of active reference layer pictures may be derived to be 0.

When at most one picture is used for interlayer prediction for eachpicture in a coding video sequence or a number of direct referencelayers of the layer including the current picture is 1, the number ofactive reference layer pictures may be derived to be 1.

When at most one picture is used for interlayer prediction for eachpicture in a coding video sequence or a number of direct referencelayers of the layer including the current picture is 1, the number ofactive reference layer pictures may be derived to be 1 if a number ofreference layer pictures available for decoding the current picture isgreater than 0, and the number of active reference layer pictures may bederived to be 0 if the number of reference layer pictures available fordecoding the current picture is 0.

When at most one picture is used for interlayer prediction for eachpicture in a coding video sequence or a number of reference layerpictures available for interlayer prediction in the same access unit asthat of the current picture is 1, the number of active reference layerpictures may be derived to be 1.

When the information on the reference layer includes number informationindicating a number of pictures used for decoding the current picturefor interlayer prediction, the number of active reference layer picturesmay be derived to be a value specified by the number information.

Advantageous Effects

According to an embodiment of the present invention, there are provideda method of signaling layer information present in a video encodedbitstream of a multilayer structure including a temporal layer, aninterlayer prediction method, and a method of obtaining a target outputlayer.

According to another embodiment of the present invention, there areprovided a method enabling even Media Aware Network Equipment (MANE)having no entropy decoder to access layer information in a bitstream forsession negotiations and an apparatus using the same.

According to still another embodiment of the present invention, thereare provided a method of accurately identifying a number of activeinterlayer reference pictures needed for decoding a current picture forutilization in interlayer prediction, a method of obtaining a targetoutput layer and an apparatus using the same.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a videoencoding apparatus according to an embodiment.

FIG. 2 is a block diagram illustrating a configuration of a videodecoding apparatus according to an embodiment.

FIG. 3 is a conceptual diagram schematically illustrating a scalablevideo coding structure using a plurality of layers according to anembodiment of the present invention.

FIG. 4 is a table illustrating information on layer sets specified in aVPS extension.

FIG. 5 is a flowchart illustrating a video decoding method according tothe present invention.

FIG. 6 illustrates a method of deriving a number of active referencelayer pictures according to an embodiment of the present invention.

MODE FOR INVENTION

Hereinafter, embodiments of the present invention are described indetail with reference to the accompanying drawings. In describing theembodiments of the present invention, a detailed description of relatedknown elements or functions will be omitted if it is deemed to make thegist of the present invention unnecessarily vague.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, the element can be directlyconnected or coupled to another element or intervening elements. Also,when it is said that a specific element is “included,” it may mean thatelements other than the specific element are not excluded and thatadditional elements may be included in the embodiments of the presentinvention or the scope of the technical spirit of the present invention.

Although the terms “first,” “second,” etc. may be used to describevarious elements, these elements should not be limited by these terms.These terms are used only to distinguish one element from anotherelement. For example, a first element may be named a second elementwithout departing from the scope of the present invention. Likewise, asecond element may be named a first element.

Although components described in the embodiments of the presentinvention are independently illustrated in order to show differentcharacteristic functions, such a configuration does not indicate thateach component is constructed by a separate hardware constituent unit orsoftware constituent unit. That is, each component includes individualcomponents that are arranged for convenience of description, in which atleast two components may be combined into a single component or a singlecomponent may be divided into a plurality of components to performfunctions. It is to be noted that embodiments in which some componentsare integrated into one combined component and/or a component is dividedinto multiple separate components are included in the scope of thepresent invention without departing from the essence of the presentinvention.

Some constituent elements are not essential to perform the substantialfunctions in the invention and may be optional constituent elements formerely improving performance. The present invention may be embodied byincluding only constituent elements essential to implement the spirit ofthe invention other than constituent elements used for merely improvingperformance. A structure including only the essential constituentelements other than optional constituents used for merely improvingperformance also belongs to the scope of the present invention.

FIG. 1 is a block diagram illustrating a configuration of a videoencoding apparatus according to an embodiment. A scalable videoencoding/decoding method or apparatus may be realized by extension of ageneral video encoding/decoding method or apparatus that does notprovide scalability, and the block diagram of FIG. 1 illustrates anexample of a video encoding apparatus which may form a basis for ascalable video encoding apparatus.

Referring to FIG. 1, the video encoding apparatus 100 includes a motionestimation module 111, a motion compensation module 112, an intraprediction module 120, a switch 115, a subtractor 125, a transformmodule 130, a quantization module 140, an entropy encoding module 150,an dequantization module 160, an inverse transform module 170, an adder175, a filter module 180, and a reference picture buffer 190.

The video encoding apparatus 100 may encode an input picture images inan intra mode or an inter mode and output a bitstream. Intra predictionmeans an intra-picture prediction, and inter prediction means aninter-picture prediction. In the intra mode, the switch 115 is shiftedto ‘intra,’ and in the inter mode, the switch 115 is shifted to ‘inter.’The video encoding apparatus 100 may generate a prediction block for aninput block of the input picture and then encode a difference betweenthe input block and the prediction block.

In the intra mode, the intra prediction module 120 may perform spatialprediction by using a pixel value of a pre-encoded block around acurrent block to generate a prediction block.

In the inter mode, the motion estimation module 111 may obtain a regionwhich is most matched with the input block in the reference picturestored in the reference picture buffer 190 during a motion estimationprocess to derive a motion vector. The motion compensation module 112may perform motion compensation using the motion vector and thereference picture stored in the reference picture buffer 190, therebygenerating the prediction block.

The subtractor 125 may generate a residual block based on the differencebetween the input block and the generated prediction block. Thetransform module 130 may transform the residual block to output atransform coefficient. The quantization module 140 may quantize thetransform coefficient according to a quantization parameter to output aquantized coefficient.

The entropy encoding module 150 may entropy-encode a symbol according toprobability distribution based on values derived by the quantizationmodule 140 or an encoding parameter value derived in encoding, therebyoutputting a bitstream. Entropy encoding is a method of receivingsymbols having different values and representing the symbols as adecodable binary sequence or string while removing statisticalredundancy.

Here, a symbol means a syntax element as an encoding/decoding target, acoding parameter, a value of a residual signal, or the like. A codingparameter, which is a parameter necessary for encoding and decoding, mayinclude information encoded by the encoding apparatus and transferred tothe decoding apparatus, such as a syntax element, and information to beinferred during an encoding or decoding process and means informationnecessary for encoding and decoding a picture. The coding parameter mayinclude, for example, values or statistics of an intra/inter predictionmode, a movement/motion vector, a reference picture index, a codingblock pattern, presence and absence of a residual signal, a transformcoefficient, a quantized transform coefficient, a block size and blockpartition information. A residual signal may denote a difference betweenan original signal and a prediction signal, a transformed signal of thedifference between the original signal and the prediction signal, or atransformed and quantized signal of the difference between the originalsignal and the prediction signal. The residual signal may be referred toas a residual block in a block unit.

When entropy encoding is applied, a symbol having a high probability isallocated a small number of bits and a symbol having a low probabilityis allocated a large number of bits in representation of symbols,thereby reducing a size of bit strings for symbols to be encoded.Accordingly, entropy encoding may enhance compression performance ofvideo encoding.

For entropy encoding, encoding methods, such as exponential Golomb,context-adaptive variable length coding (CAVLC) and context-adaptivebinary arithmetic coding (CABAC), may be used. For example, a table usedfor performing entropy encoding, such as a variable length coding/code(VLC) table, may be stored in the entropy encoding module 150, and theentropy encoding module 150 may perform entropy encoding using thestored VLC table. In addition, the entropy encoding module 150 mayderive a binarization method of a target symbol and a probability modelof a target symbol/bin and perform entropy encoding using the derivedbinarization method or probability model.

The quantized coefficient may be dequantized by the dequantization 160and inversely transformed by the inverse transform module 170. Thedequantized and inversely transformed coefficient is added to theprediction block by the adder 175, thereby generating a reconstructedblock.

The reconstructed block is subjected to the filter module 180, and thefilter module 180 may apply at least one of a deblocking filter, asample adaptive offset (SAO), and an adaptive loop filter (ALF) to thereconstructed block or a reconstructed picture. The reconstructed blockobtained via the filter module 180 may be stored in the referencepicture buffer 190.

FIG. 2 is a block diagram illustrating a configuration of a videodecoding apparatus according to an embodiment. As described above inFIG. 1, a scalable video encoding/decoding method or apparatus may berealized by extension of a general video encoding/decoding method orapparatus that does not provide scalability, and the block diagram ofFIG. 2 illustrates an example of a video decoding apparatus which mayform a basis for a scalable video decoding apparatus.

Referring to FIG. 2, the video decoding apparatus 200 includes anentropy decoding module 210, a dequantization module 220, an inversetransform module 230, an intra prediction module 240, a motioncompensation module 250, a filter module 260, and a reference picturebuffer 270.

The video decoding apparatus 200 receives an input bitstream output fromthe encoding apparatus and decodes the bitstream in an intra mode orinter mode to output a reconstituted picture, that is, a reconstructedpicture. In the intra mode, a switch may be shifted to ‘intra,’ and inthe inter mode, the switch may be shifted to ‘inter. The video decodingapparatus 200 may obtain a residual block reconstructed from the inputbitstream, generate a prediction block, and add the residual block andthe prediction block to generate a reconstituted block, that is, areconstructed block.

The entropy decoding module 210 may entropy-decode the input bitstreamaccording to probability distribution to generate symbols including asymbol in a form of a quantized coefficient. Entropy decoding is amethod of receiving a binary sequence to generate symbols. The entropydecoding method is similar to the aforementioned entropy encodingmethod.

The quantized coefficient is dequantized by the dequantization module220 and inversely transformed by the inverse transform module 230,thereby generating a reconstructed residual block.

In the intra mode, the intra prediction module 240 may perform spatialprediction by using a pixel value of a pre-encoded block around acurrent block to generate a prediction block. In the inter mode, themotion compensation module 250 may perform motion compensation using amotion vector and a reference picture stored in the reference picturebuffer 270, thereby generating a prediction block.

The reconstructed residual block and the prediction block are added byan adder 255, and the added blocks are subjected to the filter module260. The filter module 260 may apply at least one of a deblockingfilter, an SAO, and an ALF to the reconstructed block or thereconstructed picture. The filter module 260 outputs the reconstitutedpicture, that is, the reconstructed picture. The reconstructed picturemay be stored in the reference picture buffer 270 to be used for interprediction.

Among the entropy decoding module 210, the dequantization module 220,the inverse transform module 230, the intra prediction module 240, themotion compensation module 250, the filter module 260 and the referencepicture buffer 270 of the decoding apparatus 200, components directlyrelated to video decoding, for example, the entropy decoding module 210,the dequantization module 220, the inverse transform module 230, theintra prediction module 240, the motion compensation module 250 and thefilter module 260 may be defined as a decoder or a decoding unit,separately from the other components.

Further, the decoding apparatus 200 may further include a parsing module(not shown) to parse information about an encoded video included in thebitstream. The parsing module may include the entropy decoding module210 or be included in the entropy decoding module 210. The parsingmodule may be provided as one component of the decoding unit.

FIG. 3 is a conceptual diagram schematically illustrating a scalablevideo coding structure using a plurality of layers according to anembodiment of the present invention. In FIG. 3, Group of Picture (GOP)denotes a picture group, that is, a group of pictures.

In order to transmit video data, a transmission medium is needed, andperformance thereof is different by each transmission medium accordingto various network environments. For application to various transmissionmedia or network environments, a scalable video coding method may beprovided.

The scalable video coding method is a coding method which utilizestexture information, motion information, residual signals betweenlayers, or the like to remove redundancy between layers, thus improvingencoding and decoding performance. The scalable video coding method mayprovide various scalabilities in spatial, temporal, quality andviewpoint aspects according to ambient conditions such as transmissionbit rate, transmission error rate, and system resources.

Scalable video coding may be performed by using a multi-layer structureso as to provide a bitstream applicable to various network situations.For example, the scalable video coding structure may include a baselayer in which video data is compressed and processed using a generalvideo decoding method, and also include an enhancement layer in whichvideo data is compressed and processed using both decoding informationof the base layer and a general video decoding method.

Here, a layer refers to a set of pictures and bitstreams that areclassified according to a spatial aspect (for example, picture size), atemporal aspect (for example, encoding order, picture output order andframe rate), picture quality, viewpoint, complexity, or the like.Further, the base layer may refer to a lower layer or a reference layer,and the enhancement layer may refer to a higher layer. A plurality oflayers may have dependency on each other.

Referring to FIG. 3, for example, the base layer may be defined bystandard definition (SD), 15 Hz frame rate and 1 Mbps hit rate, a firstenhancement layer may be defined by high definition (HD), 30 Hz framerate and 3.9 Mbps bit rate, and a second enhancement layer may bedefined by 4K-ultra high definition (UHD), 60 Hz frame rate and 27.2Mbps. These formats, frame rates and bit rates are provided only forillustrative purposes and may be changed and modified as needed. Also, anumber of used layers may change depending on circumstances, withoutbeing limited to the present embodiment.

For instance, when a transmission bandwidth is 4 Mbps, the firstenhancement layer HD may be transmitted at a frame rate reduced to 15 Hzor lower. The scalable video coding method may provide spatial,temporal, quality and viewpoint scalabilities using the method describedabove with reference to FIG. 3.

Scalable video coding may refer to scalable video encoding in encoding,and to scalable video decoding in decoding.

The present invention relates to a process of encoding/decoding a videoincluding a plurality of layers or views, wherein the plurality oflayers or views may be expressed as first, second, third and n-th layersor views. Although the following description will be made with referenceto a picture including a first layer and a second layer, the sameprocess may be applied to pictures including two or more layers orviews. The first layer may be represented as a base layer, and thesecond layer as an upper layer. Further, the first layer may be alsorepresented as a reference layer, and the second layer as an enhancementlayer.

A picture/block in the first layer corresponding to a second-layerpicture/block may be adjusted to a size of the second-layerpicture/block. That is, if a size of the first-layer picture/block issmaller than the size of the second-layer picture/block, the first-layerpicture/block may be scaled using up-sampling or re-sampling.

The first-layer picture may be added to a reference picture list for thesecond layer and used for encoding/decoding a second-layer video. Here,the second layer may be subjected to prediction and encoding/decodingusing the first-layer picture in the reference picture list, as ingeneral inter prediction.

A block for encoding/decoding may have a square shape with an N×N size,for example, 4×4, 8×8, 16×16, 32×32 and 64×64, or a rectangular shapewith an N×M size, for example, 4×8, 16×8 and 8×32, and a block unit maybe at least one of a coding block (CB), a prediction block (PB) and atransform block (TB), which may have different sizes.

Hereinafter, a method of generating a prediction block, that is, aprediction signal, of an encoding/decoding target block (“current block”or “target block”) in an upper layer will be described in a method ofencoding and decoding a scalable video, that is, a video using amulti-layer structure. The following method or apparatus may begenerally applied to both an encoding apparatus and a decodingapparatus.

Meanwhile, according to a current draft of Scalable High EfficiencyVideo Coding (SHVC) and Multiview-High Efficiency Video Coding (MV-HEVC)standards, profile_tier_level specifying a profile, tier and level usedfor a layer set in a video parameter set (VPS) extension is described inTable 1.

TABLE 1 vps_extension( ) { Descriptor ... vps_num_profile_tier_level_minus1 u(6)  for(i=1;i<=vps_num_profile_tier_level_minus1; i++){   vps_profile_present_flag[i ] u(1)   if(!vps_profile_present_flag[ i ])    profile_ref_minus1[ i ]u(6)   profile_tier_level(vps_profile_present_flag[ i ],  vps_max_sub_layers_minus1)  }

Referring to Table 1, a value specified byvps_num_profile_tier_level_minus1 indicates a number ofprofile_tier_level( ) syntax structures in a VPS.

vps_profile_present_flag[i] equal to 1 indicates that profile and tierinformation is present in an i-th profile_tier_level( ) syntaxstructure, and vps_profile_present_flag[i] equal to 0 indicates that theprofile and tier information is not present in the i-thprofile_tier_level( ) syntax structure but is inferred.

profile_ref_minus1[i] indicates that the profile and tier informationfor the i-th profile_tier_level( ) sytanx structure is inferred to beequal to profile and tier information for a (profile_ref_minus1[i]+1)-thprofile_tier_level( ) syntax structure. Here, profile_ref_minus1[i] plus1 is less than or equal to i.

According to the current draft of the standards in Table 1, when i is 1and vps_profile_present_flag[1] is 0, profile and tier information for afirst profile_tier_level( ) syntax structure needs to be inferred from a(profile_ref_minus1[1]+1)-th profile_tier_level( ) syntax structure.That is, profile_ref_minus1[1]+1 is required to be 1 or 0. Whenprofile_ref_minus1[1]+1 is 0, profile_ref_minus1[1] is −1, thusviolating syntax definition of profile_ref_minus1[i] encoded in u(6).

Further, when (profile_ref_minus1[1]+1) is 1, a problem may occur thatfirst profiled and tier information is required to be inferred from afirst profile_tier_level syntax structure.

To address this problem, a restriction that vps_profile_present_flag[1]needs to be always 1 for a first profile_tier_level syntax structure isadded to the semantics of the syntax. In this case, semantics ofvps_profile_present_flag[i] in Table 1 may be expressed as follows.

vps_profile_present_flag[i] equal to 1 indicates that the profile andtier information is present in the i-th profile_tier_level( ) syntaxstructure, and vps_profile_present_flag[i] equal to 0 indicates that theprofile and tier information is not present in the i-thprofile_tier_level( ) syntax structure but is inferred.vps_profile_present_flag[1] for the first profile_tier_level syntaxstructure needs to be 1.

Alternatively, a signaling method illustrated in Table 2 may beconsidered to resolve the foregoing problem.

TABLE 2 vps_extension( ) { Descriptor  avc_base_layer_flag u(1)  ... vps_num_profile_tier_level_minus1 u(6)  for(i=1;i<=vps_num_profile_tier_level_minus1; i++){   vps_profile_present_flag[i ] u(1)   profile_tier_level(vps_profile_present_flag[ i ],  vps_max_sub_layers_minus1)  }  ... }

Referring to Table 2, a value specified byvps_num_profile_tier_level_minus1 indicates a number ofprofile_tier_level( ) syntax structures in a VPS.

vps_profile_present_flag[i] equal to 1 indicates that profile and tierinformation is present in an i-th profile_tier_level( ) syntaxstructure, and vps_profile_present_flag[i] equal to 0 indicates that theprofile and tier information is not present in the i-thprofile_tier_level( ) syntax structure but is inferred from profile andtier information on an (i−1)-th profile_tier_level( ) syntax structure.vps_profile_present_flag[1] for a first profile_tier_level syntaxstructure needs to be 1.

According to Table 2, profile_ref_minus1[1] is not signaled.

Alternatively, syntax structures of the VPS may be modified so that evenMedia Aware Network Equipment (MANE) having no entropy decoder may parsethe VPS extension. Tables 3 to 5 illustrate VPSs according to variousembodiments of the present invention.

TABLE 3 video_parameter_set_rbsp( ) { Descriptor vps_video_parameter_set_id u(4)  vps_reserved_three_2bits u(2) vps_max_layers_minus1 u(6)  vps_max_sub_layers_minus1 u(3) vps_temporal_id_nesting_flag u(1)  vps_extension_offset //vps_reserved_0xffff_16bits u(16)  profile_tier_level(1,vps_max_sub_layers_minus1)  vps_sub_layer_ordering_info_present_flagu(1)  for(i=(vps_sub_layer_ordering_info_present_flag  ?  0  :  vps_max_sub_layers_minus1) ;   i<= vps_max_sub_layers_minus1; i++){  vps_max_dec_pic_buffering_minus1[ i ] ue(v)  vps_max_num_reorder_pics[ i ] ue(v)   vps_max_latency_increase_plus1[i ] ue(v)  }  vps_max_layer_id u(6)  vps_num_layer_sets_minus1 ue(v) for(i=1 ; i<= vps_num_layer_sets_minus1; i++){   for(j=0 ; j<=vps_max_layer_id ; j++){    layer_id_included_flag[ i ][ j ] u(1)  ... vps_extension_flag u(1)  if (vps_extension_flag) {   while(!byte_aligned( ))    vps_extension_alignment_bit_equal_to_one u(1)  vps_extension( )   vps_extension2_flag u(1)   if(vps_extension2_flag)   while(more_rbsp_data( ))     vps_extention_data_flag u(1)  } rbsp_trailing_bits( ) }

TABLE 4 vps_extension( ) { Descriptor  avc_base_layer_flag u(1)  ... vps_num_layer_sets_minus1 u(10)  vps_num_profile_tier_level_minus1 u(6) for(i=1; i<=vps_num_profile_tier_level_minus1; i++){  vps_profile_present_flag[ i ] u(1)   if(!vps_profile_present_flag[ i])    profile_ref_minus1[ i ] u(6)  profile_tier_level(vps_profile_present_flag[ i ],  vps_max_sub_layers_minus1)  }  NumOutputLayerSets = vps_number_layer_sets_minus1 + 1 more_output_layer_sets_than_default_flag u(1) if(more_output_layer_sets_than_default_flag){  num_add_output_layer_sets_minus1 u(10)   numOutputLayerSets +=  num_add_output_layer_sets_minus1 + 1  }  if(numOutputLayerSets > 1)  default_one_target_output_layer_idc u(2)  for(i=1 ;i<numOutputLayerSets ; i++){   if(i>vps_number_layer_sets_minus1){   output_layer_set_idx_minus1[ i ] u(v)    lsIdx =output_layer_set_idx_minus1[ i ] + 1     for(j=0 ;j<NumLayersInIdList[lsIdx]−1 ; j++)      output_layer_flag[ i ][ j ]u(1)   }   profile_level_tier_idx[ i ] u(v)  }

TABLE 5 vps_extension( ) { Descriptor  ave_base_layer_flag u(1) vps_vui_present_flag u(1)  if(vps_vui_present_flag)   vps_vui_offsetu(16)  ... u(1)  all_ref_layers_active_flag u(1)  vps_maximum_layer_idu(1)  vps_num_layer_sets_minus1 u(10)  for(i=1 ; i<=vps_num_layer_sets_minus1 ; i++)   for(j=0 ; j<= vps_maximum_layer_id ;j++)    layer_id_nuh_included_flag[ i ][ j ] u(1) vps_num_profile_tier_level_minus1 u(6)  for(i=1;i<=vps_num_profile_tier_level_minus1; i++){   vps_profile_present_flag[i ] u(1)   if(!vps_profile_present_flag[ i ])    profile_ref_minus1[ i ]u(6)   profile_tier_level(vps_profile_present_flag[ i ],  vps_max_sub_layers_minus1)  }  NumOutputLayerSets = vps_number_layer_sets_minus1 + 1 more_output_layer_sets_than_default_flag u(1) if(more_output_layer_sets_than_default_flag){  num_add_output_layer_sets_minus1 u(10)   numOutputLayerSets +=  num_add_output_layer_sets_minus1 + 1  }  if(numOutputLayerSets > 1)  default_one_target_output_layer_idc u(2)  for(i=1 ;i<numOutputLayerSets ; i++){   if(i>vps_number_layer_sets_minus1){   output_layer_set_idx_minus1[ i ] u(v)    lsIdx =output_layer_set_idx_minus1[ i ] + 1    for(j=0 ;j<NumLayersInIdList[lsIdx]−1 ; j++)     output_layer_flag[ i ][ j ] u(1)  }   profile_level_tier_idx[ i ] u(v)  } ... }

Referring to Table 3, vps_extension_offset transmitted via a VPSspecifies a byte offset from a start point of a VPS NAL unit to fixedlength coded information starting with syntax avc_base_layer_flag.

The byte offset defined by vps_extension_offset enables access to piecesof basic information in the VPS NAL unit which does not need entropydecoding and enables session negotiations.

For example, the MANE having no entropy decoder may parse pieces ofbasic information not needing entropy decoding to use for sessionnegotiations based on the byte offset value specified byvps_extension_offset.

When the MANE having no entropy decoder parses output layer setsinformation in a VPS extension of Table 4 based on vps extension offsetinformation for session negotiations without entropy-decodinginformation after vps_extension_offset, NumLayersInIdList as a variablespecifying a number of layers in a layer identifier (ID) list needs tobe entropy-decoded as a value calculated from information on layer setsspecified after vps_extension_offset in Table 3, that is,layer_id_included_flag[i][j].

To make the output layer sets information in the VPS extension availablefor session negotiations without entropy decoding, information on layersets may be specified in a VPS extension of Table 5.

Meanwhile, semantics of syntax elements relating to the layer setsspecified in the VPS extension of Table 5 are as follows.

vps_maximum_layer_id, which is the same as vps_max_layer_id specified inthe VPS, specifies a maximum allowed value of nuh_layer_id of all NALunits in a coding video sequence (CVS) and may have the same value asvps_max_layer_id described in the VPS.

vps_number_layer_sets_minus1 specifies a number of layer sets and may besignaled prior to vps_vui_offset.

Similar to layer_id_included_flag[i][j] specified in the VPS,layer_id_nuh_included_flag[i][j] equal to 1 indicates that a value ofnuh_layer_id equal to j is included in a layer identifier list,layerSetLayerIdList[i], and layer_id_nuh_included_flag[i][j] equal to 0indicates that the value of nuh_layer_id equal to j is not included inlayerSetLayerIdList[i]. layer_id_nuh_included_flag[i][j] is required tohave the same value as layer_id_included_flag[i][j] specified in theVPS.

numLayersInIdList[i] and layerSetLayerIdList[i] may be obtained asfollows, i being in a range of 1 to vps_number_layer_sets_minus1.

n=0 for ( m = 0; m <= vps_maximum_layer_id; m++)    if(layer_id_nuh_included_flag[i][m])      layerSetLayerIdList[i][n++] = mnumLayersInIdList[i] = n

In multilayer-based video encoding and decoding methods, on the basis oflayer_id_nuh_included_flag[i][j] specified in the VPS extension, a VPSvideo usability information (VUI) bitstream partition hypotheticalreference decoder (HRD) parameter syntax, a bitstream partition HRDparameter supplemental enhancement information (SEI) message syntax andthe like are specified or layer sets relating information isinterpreted.

Alternatively, information on layers sets may be specified in a VPSextension of Table 6.

TABLE 6 vps_extension( ) { Descriptor  ave_base_layer_flag u(1) vps_maximum_layer_id u(1)  vps_num_layer_sets_minus1 u(10)  for(i=1 ;i<= vps_num_layer_sets_minus1 ; i++)   for(j=0 ; j<=vps_maximum_layer_id ; j++)    layer_id_nuh_included_flag[ i ][ j ] u(1) for(i=1 ; i<= MaxLayersMinus1 ; i++)   for(j=0 ; j<i ; j++)   direct_dependency_flag[ i ][ j ] u(1)  vps_vui_present_flag u(1) if(vps_vui_present_flag)   vps_vui_offset u(16)  splitting_flag u(1) for(i=0; NumScalabilityTypes=0 ; i<16 ; i++){   sealability_mask_flag[i ] u(1)   NumScalabilityTypes += scalability_mask_flag[ i ]  }  for(j=0; j<(NumScalabilityTypes-splitting_flag) ; j++)  dimension_id_len_minus1[ j ] u(3)  vps_nuh_layer_id_present_flag u(1)  ... }

Table 6 illustrates the information on the layer sets in the VPSextension, wherein session negotiations may be performed, withoutentropy decoding, using the output layer sets information in the VPSextension.

Layer sets relating syntax elements, vps_maximum_layer_id,vps_number_layer_sets_minus1 and layer_id_nuh_included_flag[i][j], maybe specified prior to vps_vui_offset.

In addition, direct dependency flag indicating dependency between layersmay be repositioned before vps_vui_offset. In this case, vps_vuiinformation may be identified using vps_vui_offset without parsing asyntax element after vps_vui_offset.

Alternatively, information on layer sets tray be specified in a VPSextension as depicted in FIG. 4

Referring to FIG. 4, layer sets relating syntax elements in the VPS maybe positioned prior to vps_extension_offset.

vps_num_layer_sets_minus1, which is conventionally encoded in variablebit ue(V), may be encoded in fixed bit u(10) to avoid entropy decoding,and vps_number_layer_sets_minus1 having the same function specified inthe VPS extension may be deleted.

Meanwhile, video signaling information specified in VPS VUI is availablefor session negotiations, and the VPS VUI may be described in Table 7.

TABLE 7 vps_vui ( ) { Descriptor  cross_layer_pic_type_aligned_flag u(1) if(!cross_layer_pic_type_aligned_flag)   cross_layer_irap_aligned_flagu(1)  bit_rate_present_vps_flag u(1)  pic_rate_present_vps_flag u(1)  ...  ilp_restricted_ref_layers_flag u(1) if(ilp_restricted_ref_layers_flag)   for(i=1 ; i<=MaxLayersMinus1 ;i++)    for(j=0 ; j<=NumDirectRefLayers[ layer_id_in_nuh    [ i ]] ;j++){     min_spatial_segment_offset_plus1[ i ][ j ] ue(v)     if(min_spatial_segment_offset_plus1[ i ]      [ j ]>0){      ctu_based_offset_enabled_flag[ i ][ j ] u(1)      if(ctu_based_offset_enabled_flag[ i ][ j ])       min_horizontal_ctu_offset_plus1[ i ][ j ] ue(v)      }     } video_signal_info_idx_present_flag u(1) if(video_signal_info_idx_present_flag)  vps_num_video_signal_info_minus1 u(4)  for(i=0 ; i<=vps_num_video_signal_info_minus1 ; i++)   video_signal_info( ) if(video_signal_info_idx_present_flag &&vps_num_video_signal_info_minus1>0)   for(i=1 ; i<= MaxLayersMinus1 ;i++)    vps_video_signal_info_idx[ i ] u(4)  ... }

Referring to Table 7, video_signal_info_idx_present_flag equal to 1indicates that vps_num_video_signal_info_minus1 andvps_video_signal_info_idx[i] are present, andvideo_signal_info_idx_present_flag equal to 0 indicates thatvps_num_video_signal_info_minus1 and vps_video_signal_info_idx[i] areabsent.

vps_num_video_signal_info_minus1 plus 1 indicates a number ofvideo_signal_info( ) syntax structures in a VPS. In the absence ofvps_num_video_signal_info_minus1, a number ofvps_num_video_signal_info_minus1 is inferred to be equal to aMaxLayersMinus1 value.

vps_video_signal_info_idx indicates an index of a video_signal_info( )syntax structure list applied to a layer having nuh_layer_id equal tolayer_id_in_nuh[i]. In the absence of vps_video_signal_info_idx,vps_video_signal_info_idx[i] is inferred as(video_signal_info_idx_present_flag ? 0 : i),vps_video_signal_info_idx[i] may be in a range of 0 tovps_num_video_signal_info_minus1.

In the current draft of the SHVC and MV-HEVC standards, since syntaxelements encoded in exponential-Golomb code (ue(v)) are present beforevideo signaling information as in Table 7, the MANE having no entropydecoder may be unable to use the video signaling information for sessionnegotiations.

To solve such a problem, that is, to use the video signaling informationin the VPS VUI for session negotiations without entropy decoding, thevideo signaling information may be specified at a position accessiblewithout entropy decoding in Table 8.

TABLE 8 vps_vui ( ) { Descriptor  cross_layer_pic_type_aligned_flag u(1) if(!cross_layer_pic_type_aligned_flag)   cross_layer_irap_aligned_flagu(1)  bit_rate_present_vps_flag u(1)  pic_rate_present_vps_flag u(1) if(bit_rate_present_vps_flag ||  pic_rate_present_vps_flag)   for(i=0 ;i<=vps_num_layer_sets_minus1 ; i++)    for(j=0 ;j<=vps_max_sub_layers_minus1 ; j++){     if(bit_rate_present_vps_flag)     bit_rate_present_flag[ i ][ j ] u(1)    if(pic_rate_present_vps_flag)      pic_rate_present_flag[ i ][ j ]u(1)     if(bit_rate_present_flag[ i ][ j ]){      avg_bit_rate[ i ][ j] u(16)      max_bit_rate[ i ][ j ] u(16)     }    if(pic_rate_present_flag[ i ][ j ]){      constant_pic_rate_idc[ i][ j ] u(2)      avg_pic_rate[ i ][ j ] u(16)     }    } video_signal_info_idx_present_flag u(1) if(video_signal_info_idx_present_flag)  vps_num_video_signal_info_minus1 u(4)  for(i=0 ; i<=vps_num_video_signal_info_minus1 ; i++)   video_signal_info( ) if(video_signal_info_idx_present_flag     &&vps_num_video_signal_info_minus1>0)   for(i=1 ; i<= MaxLayersMinus1 ;i++)    vps_video_signal_info_idx[ i ] u(4)

As in Table 8, to access the video signaling information without entropydecoding, syntax elements relating to the video signaling informationmay be described in VPS_VUI following syntax elements relating tobit_rate and pic_rate, such as bit_rate_present_vps_flag,pic_rate_present_vps_flag, bit_rate_present_flag andpic_rate_present_flag.

That is, flag information indicating presence of a signal indicating anumber of pieces of video signaling information video_signal_info and anindex of video signaling information, that is,video_signal_info_idx_present_flag, is received following signalssignaled using fixed bits, thereby accessing the video signalinginformation without entropy decoding.

Meanwhile, an aspect of the present invention suggests various methodsfor acquiring a number of active reference layer pictures used to decodea current picture for interlayer prediction.

First Method

NumActiveRefLayerPics, a variable specifying a number of activereference layer pictures used to decode a current picture for interlayerprediction, may be obtained as follows. According to a first method, allslices of a picture are defined to have the same NumActiveRefLayerPicsvalue.

(1) If nuh_layer_id as a layer ID of a layer including a current pictureis 0 or NumDirectRefLayers as a number of direct reference layers of thelayer including the current picture is 0, NumActiveRefLayerPics may beset equal to 0. That is, if the layer is a base layer or the number ofdirect reference layers is 0, the number of active reference layerpictures used to decode the current picture is set equal to 0.

(2) Otherwise, if all_ref_layers_active_flag specified in a VPSextension is 1, NumActiveRefLayerPics may be set equal to anumRefLayerPics value obtained by Equation 1, Equation 2 or Equation 3.

all_ref_layers_active_flag equal to 1 indicates that for each picturereferring to the VPS, the direct reference layer pictures that belong toall direct reference layers of the layer containing the picture, andthat may be used for inter-layer prediction as specified by the valuesof sub_layers_vps_max_minus1[i] and max_tid_il_ref_pics_plus1[i][j] arepresent in the same access unit as the picture and are included in theinter-layer reference picture set of the picture.

all_ref_layers_active_flag equal to 0 indicates that the foregoingrestriction may be applied or not be applied.

all_ref_layers_active_flag may also be expressed asdefault_ref_layers_active_flag.

numRefLayerPics, a variable indicating a number of reference layerpictures in the same AU unit as that of the current picture which areavailable for interlayer prediction may be derived as follows.

  <Equation 1>   for(i=0, j=0 ; i<NumDirectRefLayers[nuh_layer_id];i++){    refLayerIdx = LayerIdxInVps[RefLayerId[nuh_layer_id][i]]   if((sub_layers_vps_max_minus1[refLayerIdx]>=TemporalId)&&    (max_tid_il_ref_pics_plus1[refLayerIdx][LayerIdxInVps[nuh_layer_id]]>TemporalId))     refLayerPicIdc[j++] = i  }   numRefLayerPics = j

Referring to Equation 1, variable NumDirectRefLayers[ ] specifies anumber of direct reference layers of a current layer, calculated fromdirect_dependency_flag specified in the VPS extension.

sub_layers_vps_max_minus1[i] specifies maximum temporal sub-layerinformation on each layer, max_tid_il_ref_pics_plus1[i][j] specifiesmaximum allowed value of temporal sub-layer allowing inter-layerprediction in each layer, and TemporalId specifies a temporal level ofthe current picture.

According to Equation 1, among the direct reference layers of the layerincluding the current picture, only pictures in a reference layer withsub_layers_vps_max_minus1[i] greater than or equal to TemporalId of thecurrent picture and with ‘max_tild_il_ref_pics_plus1[i][j] for a currentlayer greater than TemporalId of the current picture may be consideredas direct reference layer pictures available for decoding the currentpicture for interlayer prediction.

Meanwhile, when max_tid_il_ref_pics_plus1[i][j] is 0, a non-intra randomaccess point (non-IRAP) picture with nuh_layer_id equal tolayer_id_in_nuh[i] is unavailable as a reference picture for interlayerprediction for a picture with nuh_layer_id equal to layer_id_in_nuh[j].To apply such a restriction, Equation 1 may be replaced with Equation 2.

  <Equation 2>   for(i=0, j=0 ; i<NumDirectRefLayers[nuh_layer_id];i++){    refLayerIdx = LayerIdxInVps[RefLayerId[nuh_layer_id][i]]   refLayerPicFlag =   ((sub_layers_vps_max_minus1[refLayerIdx]>=TemporalId) &&((max_tid_il_ref_pics_plus1[refLayerIdx][LayerIdxInVps[nuh_layer_id]]==0?(max_tid_il_ref_pics_plus1[refLayerIdx][LayerIdxInVps[nuh_layer_id]]==TemporalId)         :(max_tid_il_ref_pics_plus1[refLayerIdx][LayerIdxInVps[nuh_layer_id]]>TemporalId)))    if(refLayerPicflag)    refLayerPicIdc[j++] = i   }   numRefLayerPics = j

In Equation 2, variable NumDirectRefLayers[ ] specifies a number ofdirect reference layers of a current layer, calculated fromdirect_dependency_flag specified in the VPS extension.

sub_layers_vps_max_minus1[i] specifies maximum temporal sub-layerinformation on each layer, max_tid_il_ref_pics_plus1[i][j] specifiesmaximum allowed value of temporal sub-layer allowing inter-layerprediction in each layer, and TemporalId specifies a temporal level ofthe current picture.

According to Equation 2, when max_tid_il_ref_pics_plus1[i][j] is 0, onlypictures in a reference layer with max_tid_il_ref_pics_plus1[i][j] of 0equal to TemporalId of the current picture and withsub_layers_vps_max_minus1[i] greater than or equal to TemporalId of thecurrent picture, among the direct reference layers of the layerincluding the current picture, may be considered as reference layerpictures available for decoding the current picture for interlayerprediction. In this case, the pictures in the reference layer may berestricted to IRAP pictures.

When max_tid_il_ref_pics_plus1[i][j] is greater than 0, only pictures ina reference layer with sub_layers_vps_max_minus1[i] of a reference layergreater than or equal to TemporalId of the current picture and withmax_tild_il_ref_pics_plus1[i][j] of a reference layer greater thanTemporalId of the current picture may be considered as reference layerpictures available for decoding the current picture for interlayerprediction.

Alternatively, when max_tid_il_ref_pics_plus1[i][j] is 0, a non-IRAPpicture with nuh_layer_id equal to layer_id_in_nuh[i] is unavailable asa reference picture for interlayer prediction for a picture withnuh_layer_id equal to layer_id_in_nuh[j]. To apply such a restriction,Equation 1 may be replaced with Equation 3.

  <Equation 3>   for(i=0, j=0 ; i<NumDirectRefLayers[nuh_layer_id];i++){    refLayerIdx = LayerIdxInVps[RefLayerId[nuh_layer_id][i]]   if((sub_layers_vps_max_minus1[refLayerIdx]>=TemporalId)&&    (max_tid_il_ref_pics_plus1[refLayerIdx][LayerIdxInVps[nuh_layer_id]]>TemporalId || TemporalId==0))    refLayerPicIdc[j++] = i   }   numRefLayerPics = j

In Equation 3, variable NumDirectRefLayers[ ] specifies a number ofdirect reference layers of a current layer, calculated fromdirect_dependency_flag specified in the VPS extension.

sub_layers_vps_max_minus1[i] specifies maximum temporal sub-layerinformation on each layer, max_tid_il_ref_pics_plus1[i][j] specifiesmaximum allowed value of temporal sub-layer allowing inter-layerprediction in each layer, and TemporalId specifies a temporal level ofthe current picture.

According to Equation 3, only when sub_layer_vps_max_minus1[i] of areference layer is greater than or equal to TemporalId of the currentpicture which is 0 or max_tid_il_ref_pics_plus1[i][j] of the referencelayer is greater than TemporalId of the current picture, pictures in thereference layer may be considered as reference layer pictures availablefor decoding the current picture.

(3) Otherwise, if inter_layer_pred_enabled_flag specified in a slicesegment header of the current picture is 0, NumActiveRefLayerPics may beset equal to 0. inter_layer_pred_enabled_flag indicates whetherinterlayer prediction is used for decoding the current picture.

(4) Otherwise, if max_one_active_ref_layer_flag specified in the VPS is1 or NumDirectRefLayers as the number of direct reference layers of thelayer including the current picture is 1 and variable numRefLayerPicscalculated by Equation 1, Equation 2 or Equation 3 is greater than 0,NumActiveRefLayerPics may be set equal to 1. If numRefLayerPics obtainedfrom Equation 1, Equation 2 or Equation 3 is 0, NumActiveRefLayerPicsmay be set equal to 0.

max_one_active_ref_layer_flag equal to 1 indicates that at most onepicture is used for interlayer prediction of each picture in a codingvideo sequence, and max_one_active_ref_layer_flag equal to 0 indicatesthat one or more pictures are used for interlayer prediction.

(5) If conditions (1) to (4) are not satisfied, NumActiveRefLayerPicsmay be set equal to num_inter_layer_ref_pics_minus1, transmitted via theslice segment header, plus 1.

(6) When nuh_layer_id of the layer is k and TemporalId of a temporalsub-layer ID is m, numRefLayerPics in (1) to (5) may be expressed asnumRefLayerPics[k][m], which may be calculated by Equation 4 or Equation5.

Equation 1 for deriving, in a VPS level, numRefLayerPics indicating anumber of reference layer pictures available for decoding a sub-layerpicture of each layer for all layers included in a bitstream may bereplaced with Equation 4 or Equation 5. In this case,numRefLayerLayerPics may be replaced withnumRefLayerPics[nuh_layer_id][TemporalId].

<Equation 4> for(lIdx=0 ; lIdx<=MaxLayersMinus1 ; lIdx++){  lId =layer_id_in_nuh[lIdx]   for(tId=0 ; tId<=vps_max_sub_layers_minus1 ;tId++){   for(rCnt=0, k=0 ; rCnt<NumDirectRefLayers[lId]; rCnt++){   refLayerIdx=LayerIdxInVps[RefLayerId[lId][rCnt]]    if((sub_layers_vps_max_minus1[refLayerIdx]>=tId &&   (max_tid_il_ref_pics_plus1[refLayerIdx][lIdx]>tId || tId==0))     RefLayerIdListForTid[lId][tId][k++]=RefLayerId[lId][rCnt]    }   numRefLayerPics[lId][tId] = k    }  }

Equation 4, variable NumDirectRefLayers[ ] specifies a number of directreference layers of a current layer, calculated fromdirect_dependency_flag specified in the VPS extension.

sub_layers_vps_max_minus1[i] specifies maximum temporal sub-layerinformation on each layer, max_tid_il_ref_pics_plus1[i][j]′ specifiesmaximum allowed value of temporal sub-layer allowing inter-layerprediction in each layer, and vps_max_sub_layers_minus1 specifiesmaximum number of temporal sub-layers that may be presented in alllayers specified in the VPS.

In Equation 4, layer_id_in_nuh of reference layer pictures refers tonuh_layer_id present n a VCL NAL unit header.

According to Equation 4, it is determined which sub-layer is used as areference layer from a direct reference layer among sub-layers having atid (Temporal) value of 0 to vps_max_sub_layers_minus1 with respect toeach layer (0˜vps_max_layers_minus1) in a high level (for example, VPS).

As a result, in the presence of a referable sub-layer, layer_id_in_nuhof the sub-layer may be applied to RefLayerIdListForTid[[lId][tId][k++].numRefLayerPics[lId][tId] specifies a number of referable sub-layers ofa sub-layer with a tId value with respect to an lId layer.

Regarding the presence of a referable sub-layer, whensub_layers_vps_max_minus1[ ] of a reference layer is greater than orequal to TemporalId(tId) of the current picture andmax_tild_il_ref_pics_plus1[ ][ ] of the reference layer is greater thanTemporalId(tId) of the current picture which is 0, only pictures in thereference layer may be determined as reference layer pictures availablefor decoding the current picture for interlayer prediction.

<Equation 5> for(lIdx=0 ; lIdx<=MaxLayersMinus1 ; lIdx++){  lId =layer_id_in_nuh[lIdx]   for(tId=0 ; tId<=sub_layers_vps_max_minus1[lIdx]; tId++){   for(rCnt=0, k=0 ; rCnt<NumDirectRefLayers[lId]; rCnt++){    refLayerIdx=LayerIdxInVps[RefLayerId[lId][rCnt]]    if((sub_layers_vps_max_minus1[refLayerIdx]>=tId &&       (max_tid_il_ref_pics_plus1[refLayerIdx][lIdx]>tId ||       tId==0))     RefLayerIdListForTid[lId][tId][k++]=RefLayerId[lId][rCnt]    }   numRefLayerPics[lId][tId] = k   }  }

In Equation 5, variable NumDirectRefLayers[ ] specifies a number ofdirect reference layers of a current layer, calculated fromdirect_dependency_flag specified in the VPS extension.

sub_layers_vps_max_minus1[i] specifies maximum temporal sub-layerinformation on each layer, and max_tid_il_ref_pics_plus1[i][j] specifiesmaximum allowed value of temporal sub-layer allowing inter-layerprediction in each layer.

In Equation 5, layer_id_in_nuh of reference layer pictures refers tonuh_layer_id present in a VCL NAL unit header.

According to Equation 5, it is determined which sub-layer is used as areference layer from a direct reference layer among sub-layers having atid (Temporal) value of 0 to sub_layers_vps_max_minus1 of maximumtemporal sub-layer of each layer with respect to each layer(0˜vps_max_layers_minus1) in a high level (for example, VPS).

As a result, in the presence of a referable sub-layer, layer_id_in_nuhof the sub-layer may be applied to RefLayerIdListForTid[[lId][tId][k++].numRefLayerPics[lId][tId] specifies a number of referable sub-layers ofa sub-layer with a tId value with respect to an lId layer.

Regarding the presence of a referable sub-layer, whensub_layers_vps_max_minus1[ ] of a reference layer is greater than orequal to TemporalId(tId) of the current picture andmax_tild_il_ref_pics_plus1[ ][ ] of the reference layer is greater thanTemporalId(tId) of the current picture which is 0, only pictures in thereference layer may be determined as reference layer pictures availablefor decoding e current picture for interlayer prediction.

Second Method

NumActiveRefLayerPics, a number of active reference layer pictures usedto decode a current picture for interlayer prediction, may be derived asfollows. All slices of a picture are defined to have the sameNumActiveRefLayerPics.

(1) If nuh_layer_id of a layer including the current picture is 0 ornumRefLayerPics obtained by Equation 1, Equation 2 or Equation 3 is 0,NumActiveRefLayerPics may be set equal to 0.

(2) Otherwise, if all_ref_layers_active_flag specified in a VPS is 1,NumActiveRefLayerPics may be set equal to numRefLayerPics obtained byEquation 1, Equation 2 or Equation 3.

(3) Otherwise, if inter_layer_pred_enabled_flag specified in a slicesegment header of the current picture is 0, NumActiveRefLayerPics may beset equal to 0.

(4) Otherwise, if max_one_active_ref_layer_flag specified in the VPS is1 or NumDirectRefLayers specifying a number of direct reference layersof the layer including the current picture is 1, NumActiveRefLayerPicsmay be set equal to 1.

(5) If conditions (1) to (4) are not satisfied, NumActiveRefLayerPicsmay be set equal to num_inter_layer_ref_pics_minus1, transmitted via theslice segment header, plus 1.

(6) If nuh_layer_id of the layer is k and TemporalId of a temporalsub-layer ID is m, numRefLayerPics in (1) to (5) may be expressed asnumRefLayerPics[k][m], which may be derived by Equation 4 Equation 5.

Third Method

Alternatively, NumActiveRefLayerPics, a number of active reference layerpictures used to decode a current picture for interlayer prediction, maybe derived as follows. All slices of a picture are defined to have thesame NumActiveRefLayerPics.

(1) If nuh_layer_id of a layer including the current picture is 0 ornumRefLayerPics obtained by Equation 1, Equation 2 or Equation 3 is 0,NumActiveRefLayerPics may be set equal to 0.

(2) Otherwise, if all_ref_layers_active_flag specified in a VPS is 1,NumActiveRefLayerPics may be set equal to numRefLayerPics obtained byEquation 1, Equation 2 or Equation 3.

(3) Otherwise, if inter_layer_pred_enabled_flag specified in a slicesegment header of the current picture is 0, NumActiveRefLayerPics may beset equal to 0.

(4) Otherwise, if max_one_active_ref_layer_flag specified in the VPS is1 or numRefLayerPics obtained by Equation 1, Equation 2 or Equation 3 is1, NumActiveRefLayerPics may be set equal to 1.

(5) If conditions (1) to (4) are not satisfied, NumActiveRefLayerPicsmay be set equal to num_inter_layer_ref_pics_minus1, transmitted via theslice segment header, plus 1.

(6) If nuh_layer_id of the layer is k and TemporalId of a temporalsub-layer ID is m, numRefLayerPics in (1) to (5) may be expressed asnumRefLayerPics[k][m], which may be derived by Equation 4 or Equation 5.

Meanwhile, when numRefLayerPics indicating a number of reference layerpictures available to decode the current picture for interlayerprediction is derived using NumDirectRefLayers[ ] specifying a number ofdirect reference layers of a current layer, sub_layers_vps_max_minus1[i]indicating maximum temporal sub-layer information on each layer,max_tid_il_ref_pics_plus1[i][j] indicating maximum allowed value oftemporal sub-layer allowing inter-layer prediction in each layer, andTemporalId as temporal information on the current picture, calculatedfrom syntax elements specified in the VPS extension, a slice segmentheader signaling pieces of information on pictures used for interlayerprediction may be described in Table 9.

TABLE 9 slice_segment_header( ) { Descriptor  ...  if(nuh_layer_id>0 &&!all_ref_layers_active_flag &&  numRefLayerPics>0){  inter_layer_pred_enabled_flag u(1)   if(inter_layer_pred_enabled_flag&&   numRefLayerPics>1){    if(!max_one_active_ref_layer_flag)    num_inter_layer_ref_pics_minus1 u(v)    if(NumActiveRefLayerPics !=numRefLayerPics)     for(i=0; i<NumActiveRefLayerPics; i++)     inter_layer_pred_layer_idc[ i ] u(v)   }  }  ... if(sample_adaptive_offset_enabled_flag){   slice_sao_luma_flag u(1)  slice_sao_chroma_flag u(1)  }  ... }

Referring to Table 9, only when nuh_layer_id is greater than 0,all_ref_layers_active_flag specified in the VPS extension is 0 andnumRefLayerPics derived by Equation 1 or 2 is greater than 0,inter-layer_pred_enabled_flag as interlayer reference pictureinformation may be signaled.

Also, only when inter_layer_pred_enabled_flag is 1 and numRefLayerPicsis greater than 1, num_inter_layer_ref_pics_minus1 indicating a numberof interlayer reference pictures and inter_layer_pred_layer_idc[i]indicating an interlayer reference picture may be signaled.

Under the foregoing conditions, when max_one_active_ref_layer_flagspecified in the VPS extension is 1, num_inter_layer_ref_pics_minus1indicating the number of interlayer reference pictures may not besignaled.

Under the foregoing conditions, when NumActiveRefLayerPics is equal tonumRefLayerPics, inter_layer_pred_layer_idc[i] indicating the interlayerreference picture may not be signaled.

inter_layer_pred_layer_idc[i] may have a value in a range of 0 toNumDirectRefLayers−1 of the layer including the current picture, andinter_layer_pred_layer_idc[i] may be inferred to be equal torefLayerPicIdc[i] derived by Equation 1 or 2 if not signaled.

Here, information on an active reference layer picture available fordecoding the current picture may be derived by Equation 6. nuh_layer_idis nuh_layer_id of the current picture, and RefLayerId[ ][ ] islayer_id_in_nuh[ ] of a reference layer.

for(i=0, j=0; i<NumActiveRefLayerPics; i++)

RefPicLayerId[i]=RefLayerId[nuh_layer_id][inter_layer_pred_layer_idc[i]  <Equation6>

Alternatively, when numRefLayerPics is derived using Equation 4 orEquation 5, a slice segment header signaling pieces of information onpictures used for interlayer prediction may be described in Table 10.

In Table 10, nuh_layer_id is a layer ID specified in an NAL header of acurrent decoding target picture, and TemporalId is temporal informationon the current decoding target picture, that is, sub-layer layerinformation.

TABLE 10 slice_segment_header( ) { Descriptor  ...  if(nuh_layer_id>0 &&!all_ref_layers_active_flag &&    numRefLayerPics[nuh_layer_id][TemporalId]>0){  inter_layer_pred_enabled_flag u(1)   if(inter_layer_pred_enabled_flag&&     numRefLayerPics[nuh_layer_id][TemporalId]>1){   if(!max_one_active_ref_layer_flag)    num_inter_layer_ref_pics_minus1 u(v)    if(NumActiveRefLayerPics !=    numRefLayerPics[nuh_layer_id][TemporalId])     for(i=0;i<NumActiveRefLayerPics; i++)      inter_layer_pred_layer_idc[ i ] u(v)  }  }  ...  if(sample_adaptive_offset_enabled_flag){  slice_sao_luma_flag u(1)   slice_sao_chroma_flag u(1)  }  ... }

Referring to Table 10, only when nuh_layer_id is greater than 0,all_ref_layers_active_flag specified in the VPS extension is 0,numRefLayerPics[nuh_layer_id][TemporalId] derived by :Equation 4 or 5 isgreater than 0, inter-layer_pred_enabled_flag as interlayer referencepicture information may be signaled.

Further, only when inter_layer_pred_enabled_flag is 1 andnumRefLayerPics[nuh_layer_id][TemporalId] is greater than 1,num_inter_layer_ref_pics_minus1 indicating a number of interlayerreference pictures and inter_layer_pred_layer_idc[i] indicating aninterlayer reference picture may be signaled.

Under the foregoing conditions, when max_one_active_ref_layer_flagspecified in the VPS extension is 1, num_inter_layer_ref_pics_minus1indicating the number of interlayer reference pictures may not besignaled. num_inter_layer_ref_pics_minus1 may have a value in a range of0 to numRefLayerPics[nuh_layer_id][TemporalId]−1 derived by Equation 4or 5.

Under the foregoing conditions, when NumActiveRefLayerPics is equal tonumRefLayerPics[nuh_layer_id][TemporalId], inter_layer_pred_layer_idc[i]indicating the interlayer reference picture may not be signaled.

inter_layer_pred_layer_idc[i] may have a value in a range of 0 tonumRefLayerPics[nuh_layer_id][TemporalId] of the layer including thecurrent picture −1, and inter_layer_pred_layer_idc[i] may be inferred tobe equal to index ‘i’ if not signaled.

Here, information on an active reference layer picture available fordecoding the current picture may be derived by Equation 7. nuh_layer_idis nuh_layer_id of the current picture, and RefLayerIdListForTid[ ][ ]is a variable having a value of layer_id_in_nuh[ ] of a reference layerderived by Equation 4 or Equation 5.

  <Equation 7> for(i=0, j=0; i<NumActiveRefLayerPics; i++) RefPicLayerId[i] =   RefLayerIdListForTid[nuh_layer_id][TemporalId]  [inter_layer_pred_layer_idc[i]

Meanwhile, in the current draft of the SHVC and MV-HEVC standards,TargetDecLayerIdList as target decoding layer information andTargetOptLayerIdList as target output layer information are derived byEquation 8.

  <Equation 8> TargetDecLayerSetIdx =output_layer_set_idx_minus1[TargetOptLayerSetIdx]+1 lsIdx =TargetDecLayerSetIdx for(k=0, j=0; j<NumLayersInIdList[lsIdx]; j++){ TargetDecLayerIdList[j] = LayerSetLayerIdList[lsIdx][j] if(output_layer_flag[lsIdx][j])   TargetOptLayerIdList[k++] =LayerSetLayerIdList[lsIdx][j]

Referring to Equation 8, variable TargetOptLayerSetIdx indicates atarget output layer set index and may be converted into a layer setindex by output_layer_set_idx_minus1[ ] specified the VPS extension.

NumLayersInIdList specifies a number of layers included in a layer set,and TargetDecLayerIdList specifies a nuh layer id value of a layer todecode included in a layer set. TargetOptLayerIdList indicates anuh_layer_id value of a layer to output included in a layer set.

Only nuh_layer_id of a layer with output_layer_flag equal to 1 may beincluded in TargetOptLayerIdList.

output_layer_flag[ ][ ] is signaled based on a unit of output layer setin the VPS extension.

However, since output_layer_flag is determined not by output layer setbut by layer set in Equation 8, information on an output layer may notbe normally identified.

Also, since output_layer_flag[i][j] for an i-th output layer set is notspecified, i being in a range of 0 to vps_number_layer_sets_minus1,information on an output layer may not be normally identified.

To address such a problem, Equation 8 for deriving TargetDecLayerIdListas target decoding layer information TargetOptLayerIdList as targetoutput layer information may be modified into Equation 9.

output_layer_flag[i][j] of the i-th output layer set may be specifiedusing Equation 9, i being in the range of 0 tovps_number_layer_sets_minus1.

<Equation 9> TargetDecLayerSetIdx =output_layer_set_idx_minus1[TargetOptLayerSetIdx]+1 lsIdx =TargetDecLayerSetIdx for(k=0, j=0; j<NumLayersInIdList[lsIdx]; j++){ TargetDecLayerIdList[j] = LayerSetLayerIdList[lsIdx][j] if(output_layer_flag[TargetOptLayerSetIdx][j])  TargetOptLayerIdList[k++] = LayerSetLayerIdList[lsIdx][j]

In Equation 9, output_layer_flag[i][j] equal to 1 indicates that a j-thlayer in the i-th output layer set is a target output layer, andoutput_layer_flag[i][j] equal to 0 indicates the j-th layer in the i-thoutput layer set is not a target output layer.

When output layer flag indicating whether to output the layer in thej-th output layer set indicated by target output layer set indexTargetOptLayerSetIdx is equal to 1, TargetOptLayerIdList as a targetoutput layer ID list including target output layer information mayinclude a layer_id value of the j-th layer in the layer set indicated bytarget decoding layer set index TargetDecLayerSetIdx.

TargetDecLayerSetIdx may be signaled from output layer set indexinformation signaled via a VPS.

TargetDecLayerIdList as a target decoding layer ID list including targetdecoding layer information may include the layer_id value of the j-thlayer in the layer set indicated by TargetDecLayerSetIdx.

output_layer_flag[i][j] for an i-th output layer set, i being in a rangeof 0 to vps_number_layer_sets_minus1, may be inferred as below in (a)and (b), which may be stipulated in the standards.

default_one_target_output_layer_idc is signaled to derive an outputlayer in an output layer and may have a value in a range of 0 to 3.

default_one_target_output_layer_idc equal to 0 may indicate that alllayers in an output layer set are output, anddefault_one_target_output_layer_idc equal to 1 may indicate that only ahighest layer, that is, a layer with a highest layer ID, among thelayers included in the output layer set is output.

In addition, default_one_target_output_layer_idc equal to 2 may indicateonly a layer with output_layer_flag equal to 1 is output.default_one_target_output_layer_idc equal to 3 may be reserved forfuture use.

(a) When default_one_target_output_layer_idc specified in the VPS is 1,output_layer_flag[i][j] for the j-th layer included i-th layer set maybe inferred to be 1. j is set equal to NumLayersInIdList[i]−1.Otherwise, output_layer_flag[i][j] may be inferred to be 0. Here, j hasa value in a range of 0 to NumLayerInIdList[i]−1.

(b) When default_one_target_output_layer_idc specified in the VPS is 0,output_layer_flag[i][j] may be inferred to be 1. Here, j has a value ina range of 0 to NumLayerInIdList[i]−1.

vps_number_layer_sets_minus1 specified in the VPS extension indicates anumber of layer sets specified in the VPS. Since an MV-HEVC/SHVCbitstream includes two or more layer sets, vps_number_sets_minus1 isalways greater than 1. Thus, vps_number_layer_sets_minus1 encoded inu(10) may be specified to have a value in a range of 1 to 1023.Alternatively, vps_number_layer_sets_minus1 is changed intovps_number_layer_sets_minus2, which may be specified to have a value ina range of 0 to 1022.

Also, the present invention provides a method of indicating anon-reference picture unnecessary for interlayer prediction.

It may be identified based on max_tid_il_ref_pics_plus1[ ][ ] signaledvia the VPS extension whether a picture with a highest temporal ID is anon-reference picture or reference picture.

In the current draft of the SHVC and MV-HEVC standards, a picture with ahighest temporal ID is marked as a reference picture or non-referencepicture as in Equation 10.

The variable remainingInterLayerReferencesFlag is derived as specifiedin the following:

<Equation 10> remainingInterLayerReferencesFlag = 0 iLidx −LayerIdxInVps[TargetDecLayerIdList[i]]  for(j=latestDecIdx+1;j<numTargetDecLayers; j++){   jLidx =LayerIdxInVps[TargetDecLayerIdList[j]]   if(currTid<=(max_tid_il_ref_pics_plus1[iLidx][jLidx]−1))     for(k=0;k<NumDirectRefLayers[TargetDecLayerIdList[j]];k++)     if(TargetDecLayerIdList[i] ==     RefLayerId[TargetDecLayerIdList[j]][k])      remainingInterLayerReferencesFlag = 1  }

When remainingInterLayerReferenceFlag is equal to 0, currPic is markedas “unused for reference”.

In Equation 10, currTid indicates a temporal ID of a currently decodedpicture, and max_tid_il_ref_pics_plus1[iLidx][jLidx] indicates maximumtemporal ID information allowing interlayer prediction in a currentlayer, which is signaled via the VPS.max_tid_il_ref_pics_plus1[iLidx][jLidx] is signaled by upper layer withdependency on the current layer.

When the temporal ID of the currently decoded picture is less than orequal to max_tid_il_ref_pics_plus1[ ][ ] for the upper layer withdependency, remainingInterLayerReferencesFlag for upper layers withdependency on a layer including the currently decoded picture is setequal to 1.

As a result of determining remainingInterLayerReferencesFlag values ofall upper layers with dependency on the currently decoded picture, whenremainingInterLayerReferencesFlag is 0, the currently decoded picture ismarked as “non-reference picture” or “unused for reference.”

However, when the currently decoded picture is used as a reference layerfor any one of the upper layers with dependency, the currently decodedpicture is marked as “reference picture” or “used for reference.”

Thus, when remainingInterLayerReferencesFlag indicating a referencepicture for one of the upper layers with dependency is 1 in Equation 10,a process of determining remainingInterLayerReferenceFlag values of theremaining upper layers may be omitted and the currently decoded picturemay not be changed to a non-reference picture. That is, the currentlydecoded picture may be considered as a reference picture.

The variable remainingInterLayerReferencesFlag is derived as specifiedin the following:

<Equation 11> remainingInterLayerReferencesFlag = 0 iLidx −LayerIdxInVps[TargetDecLayerIdList[i]]  for(j=latestDecIdx+1;j<numTargetDecLayers           && !remainingInterLayerReferencesFlag;j++){   jLidx = LayerIdxInVps[TargetDecLayerIdList[j]]   if(currTid<=(max_tid_il_ref_pics_plus1[iLidx][jLidx]−1))     for(k=0;k<NumDirectRefLayers[TargetDecLayerIdList[j]];k++)     if(TargetDecLayerIdList[i] ==     RefLayerId[TargetDecLayerIdList[j]][k])      remainingInterLayerReferencesFlag = 1  }

When remainingInterLayerReferenceFlag is equal to 0, currPic is markedas “unused for reference”.

FIG. 5 is a flowchart illustrating a video decoding method according tothe present invention.

First, the decoding apparatus may receive information on a referencelayer used for decoding a current picture for interlayer prediction(S410).

The information on the reference layer may include flag information andinformation on numbers, such as direct_dependency_flag[i][j] indicatingwhether a layer with a j index is a direct reference layer for a layerwith an i index, sub_layers_vps_max_minus1[i] indicating maximumtemporal sub-layer information on each layer,max_tid_il_ref_pics_plus1[i][j] indicating maximum allowed value oftemporal sub-layer allowing inter-layer prediction in each layer, atemporal sub-layer ID of the current picture, all_ref_layers_active_flagindicating whether a reference layer picture available for interlayerprediction is present in the same AU as the current picture and includedin an interlayer reference picture set of the current picture, thereference layer picture being included in all direct reference layers ofa current layer including the current picture and specified by maximumtemporal sub-layer information on each layer and maximum allowed valueof temporal sub-layer allowing inter-layer prediction in each layer,inter_layer_pred_enabled_flag indicating whether interlayer predictionis used for decoding the current picture, max_one_active_ref_layer_flagindicating whether at most one picture is used for interlayer predictionfor each picture in a CVS, num_inter_layer_ref_pics_minus1 indicating anumber of pictures used for decoding the current picture for interlayerprediction or the like.

The decoding apparatus derives a number of active reference layerpictures used for decoding the current picture based on the informationon the reference layer (S420).

All slices belonging to the current picture may the same number ofactive reference layer pictures.

FIG. 6 illustrates a method of deriving a number of active referencelayer pictures according to an embodiment of the present invention. Aprocess of deriving the number of active reference layer picturesaccording to the embodiment will be described below with reference toFIG. 6.

First, it is determined whether a layer ID of a current layer includinga current picture is 0 or a number of direct reference layers of thecurrent layer including the current picture is 0 (S510).

When the layer ID of the current layer is 0 or the number of directreference layers of the current layer is 0, the number of activereference layer pictures is derived to be 0 (S520).

On the contrary, when the layer ID of the current layer is 0 or thenumber of direct reference layers of the current layer is not 0, it isdetermined whether all direct reference pictures present in the same AUas that of the current picture and included in an interlayer referencepicture set of the current picture among direct reference layer picturesincluded in all the direct reference layers of the layer including thecurrent picture are used for interlayer prediction (S530).

Operation S530 may be determined based on flag informationall_ref_layers_active_flag. When all_ref_layers_active_flag is equal to1, the number of active reference layer pictures may be derived to beequal to a reference layer picture number indicating a number ofreference layer pictures used for decoding the current picture (S540).

The reference layer picture number is derived based on a variableindicating the number of direct reference layers of the current layer,maximum temporal sub-layer information on each layer, maximum allowedvalue of temporal sub-layer allowing inter-layer prediction in eachlayer, and a temporal ID of the current picture. Here, among thepictures in the direct reference layers of the layer including thecurrent picture, pictures in a reference layer which has maximumtemporal sub-layer information greater than or equal to the temporal IDof the current picture and maximum temporal sub-layer information forthe current layer greater than the temporal ID of the current picturemay be considered as reference layer pictures available for decoding thecurrent picture for interlayer prediction.

When all_ref_layers_active_flag is 0, it is determined throughinterlayer er_pred_enabled_flag whether interlayer prediction is usedfor decoding the current picture (S550). Wheninter_layer_pred_enabled_flag is 0, the number of active reference layerpictures is derived to be 0 (S520).

Otherwise, it is determined whether at most one picture is used forinterlayer picture of each picture in a CVS or the number of directreference layers of the layer including the current picture is 1 (S560).

When max_one_active_ref_layer_flag is 1 or the number of directreference layers of the layer including the current picture is 1, thenumber of active reference layer pictures is derived to be 1 (S570).

When the foregoing determination conditions are not satisfied, thenumber of active reference layer pictures may be derived to be a valueof reference layer information specified bynum_inter_layer_ref_pics_minus1 indicating a number of pictures used fordecoding the current picture for interlayer prediction (S580).

Referring back to FIG. 5, when the number of active reference layerpictures is derived, the decoding apparatus performs interlayerprediction based on the number of active reference layer pictures(S430).

As described above, the present invention provides a method of signalinglayer information present in a video encoded bitstream of a multilayerstructure including a temporal layer, an interlayer prediction method, amethod of obtaining a target output layer and an apparatus using thesame.

The present invention also provides a method of accessing layerinformation specified in a VPS in a bitstream for session negotiationswithout an entropy decoder and an apparatus using the same.

In the aforementioned embodiments, methods have been described based onflowcharts as a series of steps or blocks, but the methods are notlimited to the order of the steps of the present invention and any stepmay occur in a step or an order different from or simultaneously as theaforementioned step or order. Further, it can be appreciated by thoseskilled in the art that steps shown in the flowcharts are not exclusiveand other steps may be included or one or more steps do not influencethe scope of the present invention and may be deleted.

The foregoing embodiments include various aspects of examples. Althoughall possible combinations to illustrate various aspects may notdescribed herein, it will be understood by those skilled in the art thatvarious combinations may be made therein without departing from thespirit and scope of the invention as defined by the appended claims.Therefore, all differences, changes and modifications within the scopewill be construed as being included in the present invention.

1. A method of decoding a video supporting a plurality of layersperformed by a decoding apparatus, the method comprising: receivinginformation on a reference layer used for decoding a current picture forinterlayer prediction; deriving a number of active reference layerpictures used for decoding the current picture based on the informationon the reference layer; performing interlayer prediction based on thenumber of active reference layer pictures to generate a prediction blockfor a current block; generating a residual block for the current block;and reconstructing the current block based on the prediction block andthe residual block, wherein the generating the residual block comprisesentropy-decoding a bitstream to generate a quantized transformedcoefficient, inverse-quantizing the quantized transformed coefficient togenerate a transformed coefficient and inverse-transforming thetransformed coefficient, wherein when a layer identifier of a currentlayer comprising the current picture is not 0 and when a number ofreference layer pictures available for interlayer prediction in the sameaccess unit as that of the current picture is not 0, and when areference layer picture available for interlayer prediction specified byvalues of maximum temporal sub-layer information on each layer andinformation on maximum allowed value of temporal slab-layer allowinginter-layer prediction in each layer among direct reference layerpictures comprised in all the direct reference layers of the currentlayer is present in the same access unit as that of the current pictureand included in an interlayer reference picture set of the currentpicture, the number of active reference layer pictures is derived to beequal to the number of reference layer pictures, and wherein the numberof reference layer pictures is derived based on information indicating anumber of direct reference layers of the current layer, the maximumtemporal sub-layer information on each layer, the information on maximumallowed value of temporal sub-layer allowing inter-layer prediction ineach layer and a temporal identifier of the current picture, and amongthe pictures in the direct reference layers of the current layercomprising the current picture, a picture in a reference layer isconsidered as a reference layer picture available for decoding thecurrent picture for interlayer prediction when maximum temporalsub-layer information of the reference layer is greater than or equal tothe temporal identifier of the current picture and either wheninformation on maximum allowed value of temporal sub-layer allowinginter-layer prediction in the reference layer for the current layer isgreater than the temporal identifier of the current picture or when thetemporal identifier of the current picture is
 0. 2. The method of claim1, wherein all slices of the current picture have the same number ofactive reference layer pictures.
 3. The method of claim 1, wherein whena layer identifier of a current layer comprising the current picture is0, the number of active reference layer pictures is derived to be
 0. 4.The method of claim 1, wherein when a number of reference layer picturesavailable for interlayer prediction in the same access unit as that ofthe current picture is 0, the number of active reference layer picturesis derived to be
 0. 5. The method of claim 1, wherein when interlayerprediction is not used for decoding the current picture, the number ofactive reference layer pictures is derived to be
 0. 6. The method ofclaim 1, wherein when at most one picture is used for interlayerprediction for each picture in a coding video sequence or when a numberof direct reference layers of the layer comprising the current pictureis 1, the number of active reference layer pictures is derived to be 1.7. The method of claim 1, wherein when the information on the referencelayer comprises number information indicating a number of pictures usedfor decoding the current picture for interlayer prediction, the numberof active reference layer pictures is derived to be a value specified bythe number information.
 8. An apparatus for decoding a video supportinga plurality of layers, the apparatus comprising: a decoder configured toreceive information on a reference layer used for decoding a currentpicture for interlayer prediction, derive a number of active referencelayer pictures used for decoding the current picture based on theinformation on the reference layer, perform interlayer prediction basedon the number of active reference layer pictures to generate aprediction block for a current block, generate a residual block for thecurrent block, and reconstruct the current block based on the predictionblock and the residual block, wherein the generating the residual blockcomprises entropy-decoding a bitstream to generate a quantizedtransformed coefficient, inverse-quantizing the quantized transformedcoefficient to generate a transformed coefficient andinverse-transforming the transformed coefficient, wherein when a layeridentifier of a current layer comprising the current picture is not 0and when a number of reference layer pictures available for interlayerprediction in the same access unit as that of the current picture is not0, and when a reference layer picture available for interlayerprediction specified by values of maximum temporal sub-layer informationon each layer and information on maximum allowed value of temporalsub-layer allowing inter-layer prediction in each layer among directreference layer pictures comprised in all the direct reference layers ofthe current layer is present in the same access unit as that of thecurrent picture and included in an interlayer reference picture set ofthe current picture, the number of active reference layer pictures isderived to be equal to the number of reference layer pictures, andwherein the number of reference layer pictures is derived based oninformation indicating a number of direct reference layers of thecurrent layer, the maximum temporal sub-layer information on each layer,the information on maximum allowed value of temporal sub-layer allowinginter-layer prediction in each layer and a temporal identifier of thecurrent picture, and among the pictures in the direct reference layersof the current layer comprising the current picture, a picture in areference layer is considered as a reference layer picture available fordecoding the current picture for interlayer prediction when maximumtemporal sub-layer information of the reference layer is greater than orequal to the temporal identifier of the current picture and either wheninformation on maximum allowed value of temporal sub-layer allowinginter-layer prediction in the reference layer for the current layer isgreater than the temporal identifier of the current picture or when thetemporal identifier of the current picture is
 0. 9. The apparatus ofclaim 8, wherein when a layer identifier of a current layer comprisingthe current picture is 0, the number of active reference layer picturesis derived to be
 0. 10. The apparatus of claim 8, wherein when a numberof reference layer pictures available for interlayer prediction in thesame access unit as that of the current picture is 0, the number ofactive reference layer pictures is derived to be
 0. 11. The apparatus ofclaim 8, wherein when interlayer prediction is not used for decoding thecurrent picture, the number of active reference layer pictures isderived to be
 0. 12. The apparatus of claim 8, wherein when at most onepicture is used for interlayer prediction for each picture in a codingvideo sequence or when a number of direct reference layers of the layercomprising the current picture is 1, the number of active referencelayer pictures is derived to be
 1. 13. A method of encoding a videosupporting a plurality of layers performed by an encoding apparatus, themethod comprising: determining information on a reference layer used forencoding a current picture for interlayer prediction; deriving a numberof active reference layer pictures used for encoding the current picturebased on the information on the reference layer; performing interlayerprediction based on the number of active reference layer pictures togenerate a prediction block for a current block; generating a residualblock for the current block based on the prediction block; and encodingthe residual block, wherein the encoding the residual block comprisestransforming the residual block to generate a transformed coefficient,quantizing the transformed coefficient to generate a quantizedtransformed coefficient and entropy-encoding the quantized transformedcoefficient, wherein when a layer identifier of a current layercomprising the current picture is not 0 and when a number of referencelayer pictures available for interlayer prediction in the same accessunit as that of the current picture is not 0, and when a reference layerpicture available for interlayer prediction specified by values ofmaximum temporal sub-layer information on each layer and information onmaximum allowed value of temporal sub-layer allowing inter-layerprediction in each layer among direct reference layer pictures comprisedin all the direct reference layers of the current layer is present inthe same access unit as that of the current picture and included in aninterlayer reference picture set of the current picture, the number ofactive reference layer pictures is derived to be equal to the number ofreference layer pictures, and wherein the number of reference layerpictures is derived based on information indicating a number of directreference layers of the current layer, the maximum temporal sub-layerinformation on each layer, the information on maximum allowed value oftemporal sub-layer allowing inter-layer prediction in each layer and atemporal identifier of the current picture, and among the pictures inthe direct reference layers of the current layer comprising the currentpicture, a picture in a reference layer is considered as a referencelayer picture available for decoding the current picture for interlayerprediction when maximum temporal sub-layer information of the referencelayer is greater than or equal to the temporal identifier of the currentpicture and either when information on maximum allowed value of temporalsub-layer allowing inter-layer prediction in the reference layer for thecurrent layer is greater than the temporal identifier of the currentpicture or when the temporal identifier of the current picture is
 0. 14.An apparatus for encoding a video supporting a plurality of layers, theapparatus comprising: an encoder configured to determine information ona reference layer used for encoding a current picture for interlayerprediction, derive a number of active reference layer pictures used forencoding the current picture based on the information on the referencelayer, perform interlayer prediction based on the number of activereference layer pictures to generate a prediction block for a currentblock, generate a residual block for the current block based on theprediction block, and encode the residual block, wherein the encodingthe residual block comprises transforming the residual block to generatea transformed coefficient, quantizing the transformed coefficient togenerate a quantized transformed coefficient and entropy-encoding thequantized transformed coefficient, wherein when a layer identifier of acurrent layer comprising the current picture is not 0 and when a numberof reference layer pictures available for interlayer prediction in thesame access unit as that of the current picture is not 0, and when areference layer picture available for interlayer prediction specified byvalues of maximum temporal sub-layer information on each layer andinformation on maximum allowed value of temporal sub-layer allowinginter-layer prediction in each layer among direct reference layerpictures comprised in all the direct reference layers of the currentlayer is present in the same access unit as that of the current pictureand included in an interlayer reference picture set of the currentpicture, the number of active reference layer pictures is derived to beequal to the number of reference layer pictures, and wherein the numberof reference layer pictures is derived based on information indicating anumber of direct reference layers of the current layer, the maximumtemporal sub-layer information on each layer, the information on maximumallowed value of temporal sub-layer allowing inter-layer prediction ineach layer and a temporal identifier of the current picture, and amongthe pictures in the direct reference layers of the current layercomprising the current picture, a picture in a reference layer isconsidered as a reference layer picture available for decoding thecurrent picture for interlayer prediction when maximum temporalsub-layer information of the reference layer is greater than or equal tothe temporal identifier of the current picture and either wheninformation on maximum allowed value of temporal sub-layer allowinginter-layer prediction in the reference layer for the current layer isgreater than the temporal identifier of the current picture or when thetemporal identifier of the current picture is
 0. 15. A non-transitorycomputer-readable medium storing a bitstream that is generated by amethod of encoding a video supporting a plurality of layers, the methodcomprising: determining information on a reference layer used forencoding a current picture for interlayer prediction; deriving a numberof active reference layer pictures used for encoding the current picturebased on the information on the reference layer; performing interlayerprediction based on the number of active reference layer pictures togenerate a prediction block for a current block; generating a residualblock for the current block based on the prediction block; and encodingthe residual block, wherein the encoding the residual block comprisestransforming the residual block to generate a transformed coefficient,quantizing the transformed coefficient to generate a quantizedtransformed coefficient and entropy-encoding the quantized transformedcoefficient, wherein when a layer identifier of a current layercomprising the current picture is not 0 and when a number of referencelayer pictures available for interlayer prediction in the same accessunit as that of the current picture is not 0, and when a reference layerpicture available for interlayer prediction specified by values ofmaximum temporal sub-layer information on each layer and information onmaximum allowed value of temporal sub-layer allowing inter-layerprediction in each layer among direct reference layer pictures comprisedin all the direct reference layers of the current layer is present inthe same access unit as that of the current picture and included in aninterlayer reference picture set of the current picture, the number ofactive reference layer pictures is derived to be equal to the number ofreference layer pictures, and wherein the number of reference layerpictures is derived based on information indicating a number of directreference layers of the current layer, the maximum temporal sub-layerinformation on each layer, the information on maximum allowed value oftemporal sub-layer allowing inter-layer prediction in each layer and atemporal identifier of the current picture, and among the pictures inthe direct reference layers of the current layer comprising the currentpicture, a picture in a reference layer is considered as a referencelayer picture available for decoding the current picture for interlayerprediction when maximum temporal sub-layer information of the referencelayer is greater than or equal to the temporal identifier of the currentpicture and either when information on maximum allowed value of temporalsub-layer allowing inter-layer prediction in the reference layer for thecurrent layer is greater than the temporal identifier of the currentpicture or when the temporal identifier of the current picture is 0.